1. Field of the Invention
The present invention relates to a production method of an optical waveguide to be widely used for optical communications, optical information processing, and other general optics.
2. Description of the Related Art
Optical waveguides are typically incorporated in optical devices such as optical waveguide devices, optical integrated circuits and optical wiring boards to be widely used for optical communications, optical information processing and other general optics. Such an optical waveguide typically includes an under-cladding layer, cores of a predetermined projection pattern provided as light paths on an upper surface of an under-cladding layer, and an over-cladding layer covering the cores.
The following production method is proposed as an optical waveguide production method to be employed, for example, for simultaneously producing a plurality of optical waveguides (see, for example, JP-A-2008-203694). First, as shown in FIG. 6A, a single under-cladding layer 11 having an area sufficient to accommodate a plurality of optical waveguides W0 (see FIG. 6D) is formed on a substrate A. Then, as shown in FIG. 65, a plurality of regions for the plurality of optical waveguides W0 are defined on an upper surface of the under-cladding layer 11, and cores 12 are formed in a predetermined projection pattern on each of the regions. In turn, as shown in FIG. 6C, a liquid curable resin (a photosensitive resin or a thermosetting resin) is applied as a material for an over-cladding layer 13 on the entire upper surface of the under-cladding layer as covering the cores 12, and then the resulting coating layer 13a is cured, whereby the over-cladding layer 13 is formed. Thus, the optical waveguides W0 each including the under-cladding layer 11, the cores and the over-cladding layer 13 are formed in a combined state on the substrate A. Further, as shown in FIG. 6D, the resulting substrate A is cut along boundaries between the optical waveguides W0 by a rotary blade or the like. Thereafter, the optical waveguides W0 are each separated from the substrate A. Thus, the optical waveguides W0 are produced.
In the optical waveguide production method, however, the coating layer 13a formed by applying the liquid curable resin for the formation of the over-cladding layer (see FIG. 6C) has a greater thickness on core formation areas in which the cores 12 are formed, and has a smaller thickness on areas in which the cores 12 are not formed between adjacent ones of the optical waveguides W0 and, in this state, the coating layer 13a is cured. Therefore, the over-cladding layer 13 has an undulated upper surface. If an optical waveguide W0 including an over-cladding layer 13 having a non-planar upper surface is connected to a connector, it is difficult to fix a smaller thickness portion of the optical waveguide W0 to the connector, so that the optical waveguide W0 and the connector are liable to be misaligned. This exacerbates a connection loss occurring when the optical waveguide W0 is connected to the connector. In FIG. 6C, the undulated upper surface of the over-cladding layer 13 is illustrated with exaggeration for easy understanding.
Proposed approaches to planarization of the upper surface of the over-cladding layer 13 are to polish the upper surface and to form another layer on the upper surface (see, for example, JP-A-2001-74953, JP-A-2001-74963 and JP-A-2001-91775).
However, the planarization of the upper surface of the over-cladding layer 13 requires additional steps, i.e., the step of polishing the upper surface and the step of forming another layer on the upper surface, thereby reducing the productivity. Further, the optical waveguides W0 often each have a complicated overall shape depending on their applications. This makes it difficult to separate the optical waveguides W0 from each other by cutting with the rotary blade or the like, thereby reducing the productivity.